Motor operated valves (MOVs) are used in many critical applications to control and/or isolate fluid flow. All nuclear power plants, as well as fossil, petrochemical, and other industries that handle fluids routinely apply MOVs to accomplish positive isolation of flow.
It is, of course, highly desirable to diagnose and monitor the operating condition of various electric motors, including MOVs, particularly those used in critical applications. An example of a monitoring system is described in U.S. Pat. No. 4,542,649 to Charbonneau et al. The monitoring system includes motor current measurement, and means for correlating spring pack movement to valve stem load.
Another example of a monitoring system is described in U.S. Pat. No. 4,965,513 to Haynes et al. In this system, motor current is sensed, conditioned, and analyzed in the frequency domain to diagnose operability of the motor.
Existing techniques are available for measuring torque of an ac motor while the motor is coupled to a line voltage. An example is described in U.S. Pat. No. 4,616,179 to Braun. The system described therein measures line to neutral voltage and line current and derives torque therefrom.
With respect to MOVs, a particularly important parameter associated with flow isolation is the force applied to seat the valve. The MOV's close direction stroke is terminated when a torque switch opens to deenergize the motor. The sensor used to measure torque does not measure motor torque directly; rather, it is actuated through the movement of a worm which is located several mechanical interfaces from the motor shaft. As a result, the actual motor torque is not known.
Most MOV motors are electric ac induction motors. Motor output torque can be affected by a variety of factors, such as manufacturing uncertainties, rotor condition, stator condition, line voltage, and motor temperature. MOV motors have been particularly subject to rotor degradation, due in part to the use of magnesium rotors (to minimize the rotational inertia of the motors).
The motor torque output at reduced voltage is of critical concern because of the requirement that the motor must open or close the valve in a degraded voltage condition. Unless test data is available on a specific motor, conservatism for manufacturing uncertainties and motor feeder parameters must be applied to the torque output calculation. These conservatisms can make successful motor torque calculations extremely difficult.
During the majority of the MOV stroke, the motor is only very lightly loaded. As a result, its speed is normally very near synchronous. However, during the increased load experienced by the motor during valve seating, the motor slows down slightly and its output torque increases significantly. At the moment that the torque switch trips the motor, the torque output developed by the motor is at its maximum.
If motor speed were known, it would be possible to roughly estimate the motor torque (using generic manufacturer data). However, the motor shaft is totally enclosed within the operator, and therefore, motor speed cannot be measured from external means, such as shaft gear proximity probes. Likewise, torque cannot be directly measured using conventional means, since shaft torque measuring devices require the insertion of a torque cell integral with the motor shaft.
There have been technologies developed which measure speed (based on motor current) during the steady-state portion of the stroke, but there are no existing technologies which allow the measurement of the final speed or torque at motor deenergization, which is the most important condition to understand.